Work around pypy testing issue

A recent release of setuptools appears to have introduced a bug that
causes the pypy gate tests to fail. This applies a temporary workaround
that should restore pypy testing while the root cause of the problem is
researched.

The work around is suggested in the comment in bug 1290562.

Change-Id: Iac2ec604447b1638550219cd2e407daf188f8ede
Partial-Bug: 1290562
Merge "Work around pypy testing issue"